Initial stab at background check.

Actually, this implementation is more what we want for ephemeral
apps.  I am realizing the two are not really the same thing. :(

For this implementation, we now keep track of how long a uid has
been in the background, and after a certain amount of time
(currently 1 minute) we mark it as "idle".  Any packages associated
with that uid are then no longer allowed to run in the background.
This means, until the app next goes in the foreground:

- No manifest broadcast receivers in the app will execute.
- No services can be started (binding services is still okay,
  as this is outside dependencies on the app that should still
  be represented).
- All alarms for the app are cancelled and no more can be set.
- All jobs for the app are cancelled and no more can be scheduled.
- All syncs for the app are cancelled and no more can be requested.

Change-Id: If53714ca4beed35faf2e89f916ce9eaaabd9290d